The present invention relates to a compound comprised of an intercalator or substituted intercalator segment derivatized with functionalized chains having high affinity for binding to a DNA molecule. More particularly, the invention relates to use of these intercalator compounds which exhibit improved binding to a DNA molecule within known methodologies requiring intercalator insertion into the DNA molecule. Still, the invention relates to enhanced binding of DNA molecules by an intercalator functioning segment utilized in labeling, capture, therapeutic insertion, assay and the like, with improved performance of the intercalator due to the increased utilization efficiency of the compounds.
The term "intercalator" was introduced into the chemistry field over 30 years ago to describe the insertion of planar aromatic or heteroaromatic compounds between adjacent base pairs of double stranded DNA (dsDNA). Many DNA intercalating compounds elicit biologically interesting properties. It is generally agreed that these properties are related to their reactivity with DNA. In the search for more active compounds, it is logical to design molecules with the highest possible affinity for DNA. In 1990, it was reported that complexes of ethidium homodimer with dsDNA performed at ratios of one dimer per four to five base pairs, and were stable to electrophoresis on agarose gels. This allowed fluorescence, detection and quantitation of DNA fragments with picogram sensitivity after separation and complete absence of background stain. Such a result has been sought through various manipulations of intercalator compounds, for example, by functional compounds made up of DNA intercalating dyes. As a result of these efforts, DNA intercalating agents utilizing ethidium bromide have been used in various DNA analytical procedures.
Various reported DNA intercalating agents utilizing ethidium bromide have been used in a multitude of DNA analytical procedures, for example:
Christen, et al., "An Ethidium Bromide-Agarose Plate Assay for the Nonradioactive Detection of CDNA Synthesis", Anal. Biochem., May 1, 1989, 178 (2), pp. 269-272, report ethidium bromide was used to determine the success of cDNA synthesis reactions. Since ethidium bromide in agarose can be used to quantitate RNA and DNA, conditions under which the greater fluorescence of double-stranded DNA is utilized were devised to assay double stranded DNA synthesis from mRNA. Ethidium bromide at 5 micrograms/ml in agarose allowed quantitative detection of cDNA in the range of 0.03 to 0.0015 microgram. Sodium dodecyl sulfate had an adverse effect on the measurement of cDNA. Subsequent cDNA analysis by alkaline gel electrophoresis and staining in 5 micrograms/ml ethidium bromide allowed accurate and rapid sizing of cDNA and required only 0.01-0.05 microgram cDNA.
Petersen, S. E., "Accuracy and Reliability of Flow Cytometry DNA Analysis Using a Simple, One-Step Ethidium Bromide Staining Protocol", Cytometry, July, 1986, 7 (4), pp. 301-306, reports that sources of variation and error were investigated for a simple flow cytometric analysis of DNA content of detergent-isolated nuclei stained with ethidium bromide.
In "Ethidium Bromide in the Detection of Antibodies to DNA and of Circulating DNA by Two-Stage Counterimmunoelectrophoresis", J. Immunol. Methods, Dec. 17, 1985, 85 (1), pp. 217-220, Riboldi, et al., report that in an attempt to overcome the limitations of counterimmunoelectrophoresis in the detection of precipitating anti-DNA antibodies or circulating DNA, ethidium bromide was used to increase the visibility of the precipitating lines and to confirm their specificity.
W. A. Denny reported in "DNA-Intercalating Ligands as Anti-Cancer Drugs: Prospects for Future Design", Anticancer Drug Des., December, 1989, 4 (4), pp. 241-263, that interest in DNA-intercalating ligands as anti-cancer drugs has developed greatly since the clinical success of doxsorubicin.
A number of agents have been described for labeling nucleic acids, whether probe or target, for facilitating detection of target nucleic acid. Suitable labels may provide signals detectable by fluorescence, radioactivity, colorimetry, X-ray diffraction or absorption, magnetism or enzymatic activity, and include, for example, fluorophores, chromophores, radioactive isotopes, enzymes, and ligands having specific binding partners.
Fluorescent dyes are suitable for detecting nucleic acids. For example, ethidium bromide is an intercalating agent that displays increased fluorescence when bound to double stranded DNA rather than when in free solution. Ethidium bromide can be used to detect both single and double stranded nucleic acids, although the affinity of ethidium bromide for single stranded nucleic acid is relatively low. Ethidium bromide is routinely used to detect nucleic acids following gel electrophoresis. Following size fractionation on an approximate gel matrix, for example, agarose or acrylamide, the gel is soaked in a dilute solution of ethidium bromide.
The use of fluorescence labeled polynucleotide probes and polynucleotide hybridization assays have been reported. According to these methods, probes are prepared by attaching a particular absorber-emitter moieties to the three prime and five prime ends of the nucleic acid fragments. The fragments are capable of hybridizing to adjacent positions of a target DNA so that if both fragments are hybridized, the proximity of the absorber and emitter moieties results in the detectable emitter fluorescence. According to these methods, the fluorescent dye is introduced into the target DNA after all in vitro nucleic acid polymerizations have been completed. The inhibitory effects of intercalating agents on nucleic acid polymerases have been described in numerous locations.
DNA binding dyes are useful as antibiotics because of the inhibitory effects of nucleic acid replication processes that result from the agent binding to the template. The use of intercalating agents for blocking infectivity of influenza or herpes viruses have been reported. It has also been reported and described that a number of DNA binding agents, both intercalators and nonintercalators, inhibit nucleic acid replication. For example, ethidium bromide inhibits DNA replication.
Methods have been provided for detecting a target nucleic acid in a sample. These methods comprise the steps of (a) providing an amplified reaction mixture that comprises a sample, a DNA binding agent, where said agent is characterized by providing a detectable signal when bound to double stranded nucleic acid, which signal is distinguishable from the signal provided by said agent when it is unbound, and reagents for amplification; (b) determining the amount of signal produced by the mixture of step (a); (c) treating said mixture under conditions for amplifying the target nucleic acid; (d) determining the amount of said signal produced by the mixture of step (c); and (e) determining if amplification has occurred. These DNA binding intercalating agents, such as ethidium bromide or ethidium homodimer allow fluorometric study of the interaction of various molecules with DNA.
The intercalating agent useful for DNA binding or detecting amplified nucleic acids is an agent or moiety capable of insertion between stacked base pairs in the nucleic acid double helix. Intercalating agents such as ethidium homodimer and ethidium bromide fluoresce more intensely when intercalated into double stranded DNA than when bound to single stranded DNA, RNA, or in solution. Other uses of intercalators have been in the field of separation and isolation or purification of nucleic acids from complex biological or clinical specimens.
Various methods of separating deoxyribonucleic acids (DNA) from liquid biological samples are known in the art, but are very time consuming or otherwise plagued by complication. It is known that DNA adheres to nitrocellulose. The liquid sample containing DNA is applied to a nitrocellulose filter and the DNA adheres or binds to the filter.
Another method of separating DNA from samples is ultracentrifugation with sucrose or cesium chloride density gradients. The DNA is separated from other macromolecules such as proteins by this method according to the buoyant density or sedimentation coefficient. The biological sample is layered onto the density gradient in a centrifuge tube and is spun at very high speeds for long periods of time for DNA to travel through the density gradient. This method, although satisfactory, is very time consuming and labor intensive. The centrifugation time may be 20 hours or more per sample. Furthermore, if the sample is spun too long, the DNA will separate from the sample but will pass entirely through the gradient to the very bottom of the centrifuge tube along with other constituents in the sample. Therefore, this method is also not suitable as a fast and easy method for separating DNA from complex samples.
Agarose polyacrylamide gel electrophoresis is also used to separate DNA from biological samples. In this method, the sample is applied to one end of a glass or plastic receptacle containing the gel and an electric current is applied across the length of the receptacle. The negatively charged nucleic acid molecules move toward the anode, the larger molecules moving more slowly. The rates of migration of the molecules depend on their molecular weights and on the concentration and degree of cross linking in the gel material. The DNA is then removed from the gel by cutting out that portion of the gel in which the DNA is located and finally extracting the DNA. Again, this method is time consuming and labor intensive, and the DNA must still be separated from the gel. When DNA is separated by the electrophoresis gel method or by centrifugation, it is necessary for the DNA to be stained in some manner to be visualized. Typically, ethidium bromide (EtBr) has been used as the staining agent. Ethidium bromide adheres to the DNA by intercalation between the base pairs of the double helix structure of the DNA.
More recently, an ethidium homodimer has been synthesized and introduced with bifunctional intercalators in order to allow fluorometric study including the interaction of such molecules with DNA. It has been determined that the ethidium homodimer (EthD) binds to dsDNA (double stranded DNA) about two (2) orders of magnitude more strongly than ethidium bromide. Complexes of EthD with dsDNA have performed at a ratio of one dimer per 4 to 5 base pairs and were found to be stable to electrophoresis on agarose base. On binding to dsDNA, the fluorescence quantum yield of the dimer increases 40 fold independent of nucleotide sequence.
Stable dsDNA-fluoropore complexes can be formed to obtain anywhere from several to several thousand fluoropores each, as desired. Under suitable controlled conditions these complexes do not transfer dye to other nucleic acids or proteins. An important property of these complexes is that their fluorescence emission intensity is a linear function of the number of intercalated dye molecules. As high sensitivity fluorescence detection apparatus becomes more generally available, the ability to use dyes to replace, for example, radioactivity for sensitivity detection of DNA, is becoming more and more valuable.
Dye dsDNA complexes represent a novel family of fluorescence labels with a wide range of spectroscopic properties whose composition, structure and size can be tailored to particular applications. DNA molecules can be readily derivated to attach biotin, digoxigenin or any number of other substituents that can be recognized by avidin or antibodies. Such derivatize DNA molecules loaded with dye may allow detection at much higher sensitivity in numerous applications, for example, immunoassay, fluorescence, and in situ hybridization of chromosomes and the like that currently use other fluorescence labels.
Probes with a double stranded region, which provide intercalation sites and a single stranded region to allow recognition by hybridization of specific target sequences, offer another approach to the generation of versatile fluorescent labels. Development of conditions that allow clear discrimination between the binding of intercalators to single and double stranded nucleic acids is an essential prerequisite to the use of such probes.
Fluorescent probes are valuable reagents for the analysis and separation of molecules and cells. Some specific examples of their application are identification and separation from a subpopulation of cells in a mixture of cells by the techniques of fluorescence, flow cytometry, fluorescence-activated cell sorting, and fluorescence microscopy. Other applications include determination of a concentration of a substance or member of a specific binding pair that binds to a second species, or member of the specific binding pair, e.g., antigen-antibody reactions in an immunofluorescent assay. Still another application is the localization of substance in gels and other insoluble supports by the techniques of fluorescence staining.
Choice of fluorescers for these purposes is hampered by various constraints; one being the absorption and emission characteristics of the fluorescer since many ligands, receptors and other binding pair members, as well as other extraneous materials associated with the sample, for example, blood, urine and cerebrospinal fluid, will auto-fluoresce and interfere with an accurate determination or quantification of the fluorescent signal generated by the fluorescent label when the sample is exposed to the appropriate stimulus. Another consideration is the quantum efficiency of the fluorescer. Yet another concern is self-quenching; this can occur when the fluorescent molecules interact with each other when in close proximity. An additional concern is the non-specific binding of the fluorescer to other compounds or even with the test container.
It has been shown that double stranded DNA (dsDNA) forms spectrophotometric, highly fluorescent complexes with the bis-intercalator ethidium homodimer (EthD). Observations regarding the bis-intercalator EthD suggest that the intercalator can be exploited to generate a family of highly fluorescent stable dsDNA-dye complexes with distinctive properties. Such complexes could be exploited by multiplex detection of dsDNA fragments, as well as many analytical applications in which appropriately diversified dsDNA fragments labeled noncovalently with different dyes could be used as a unique family of fluorescent probes: ##STR1## However, this compound may have a tendency to self-quench when bound to DNA. The "bis" structures inhibit each other because the "bis" intercalators cannot simultaneously intercalate.
In flow cytometry apparatuses, cells or other particles are caused to flow in a liquid flow stream so as to facilitate the investigation of certain characteristics thereof. In general, a flow cytometry apparatus is useful for identifying the presence of certain cells or particles of interest, enumerating those cells or particles and, in some instances, providing a sorting capability so as to be able to collect those cells or particles of interest. In a typical flow cytometry apparatus, a fluid sample containing cells is directed through the apparatus in a rapidly moving liquid stream so that each cell passes serially, and substantially one at a time, through a sensing region. Cell volume may be determined by changes in electrical impedance as each cell passes through the sensing region. Similarly, if an incident beam of light is directed at the sensing region, the passing cells scatter such light as they pass therethrough. This scattered light has served as a function of cell shape and size, index of refraction, opacity, granularity, roughness and the like. Further, fluorescence emitted by labeled cells, or autofluorescent cells, which have been excited as a result of passing through the excitation energy of the incident light beam is detectable for identification of cells having fluorescent properties. After cell analysis is performed by the flow cytometry apparatus, those cells that have been identified as having the desired properties may be sorted if the apparatus has been designed with such capability.
Instruments such as flow cytometry apparatuses are particularly useful for researchers and investigators studying various responses, reactions and functions of the immune system. Immunofluorescence studies, as well as fluorescence immunoassays, assist the investigator in identifying and targeting select cells of interest so that disease states, conditions and the like may be properly characterized. In addition to immune system investigations, fluorescence analysis is also quite beneficial in cell biology and morphology investigations, including the study of the substrate of cellular material.
In relying upon fluorescence to provide data and information about cells, the mechanics of performing tests for the fluorescence response is a major consideration in the design of the instrument as well as the results obtained. Specifically, the fluorescent markers, whether such markers be fluorescent stains or dyes, are typically excited by light energy. Usually there is an optimal wavelength which provides the greatest level of excitation for the fluorochromatic marker being used. Once excited, fluorescence emission occurs typically at wavelengths different from the wavelength of excitation. Fluorescence analysis instruments, whether fluorescence microscopes, image analyzers or flow cytometers, are generally designed to detect the fluorescence emission at the wavelength of emission maxima where the fluorescence signal is strongest.
Before the discovery and publication of the utilities of ethidium homodimer as an important intercalator, the usual intercalator of choice was ethidium bromide. Uses of the ethidium bromide intercalators include fluorometric methodologies, quantitative fluorescences of DNA intercalated ethidium bromide on agarose gels, ethidium bromide-agarose plate assay or detection of false DNA analysis and the like. Ethidium bromide and propidium bromide were further used in flow cytometry, as well as applications for direct electronic imaging, direct and rapid quantitation of fluorescence and electrophoretic gels in application as ethidium bromidestain DNA. Ethidium bromide has also been used to increase the visibility of the precipitant lines and to confirm the specificity in two stage counter immunoelectrophoresis methodologies for detection of participating anti-DNA antibodies or circulating DNA. Utilization of ethidium bromide as an intercalator in numerous environments, as well as the more recent utilization of the ethidium homodimer intercalator are well documented in the literature and present the leading edge of intercalator methodology and efficiency.
In a somewhat different application of ethidium bromide as a staining agent, ethidium bromide has been linked to a solid support. U.S. Pat. No. 4,119,521, issued to Chirikjian on Oct. 10, 1978, discloses a fluorescent DNA intercalating agent derivative of activated polysaccharides. The derivatives in the patent function as fluorescent stains to provide direct visualization of the DNA and their fractions, under the excitation of shortwave, ultraviolet radiation. The intercalating agents used in the patent are ethidium halides, with the preferred agent being ethidium bromide. This agent is coupled covalently to an activated polysaccharide such as agarose.
Utilization of ethidium bromide as an intercalator for use in numerous environments, as well as the more recent utilization of the ethidium homodimer intercalator are well documented in the literature and present the leading edge of intercalator methodology and efficiency. However, there remains an ever present need to improve utilization of the intercalators and viability of the use of intercalators with DNA, specifically addressing (1) high affinity for binding the intercalators to the DNA molecule; (2) reduction of self-quenching; and (3) providing superior transport kinetics. Intercalators possessing these qualities reduce the amount of intercalator required for performing one of the many functions involved in the aforementioned methodologies which can also enhance methodologies. In addition, improvement in accuracy and reliability of the various uses of interest is of continuing concern.